A pilot study of sampling, using monthly marine flights over spatially distributed stations, was conducted with the aim to characterize the carbonate system in Puget Sound over a full year-long period. Surface waters of Puget Sound were found to be under-saturated with respect to aragonite during October–March, and super-saturated during April–September. Highest pCO 2 and lowest pH occurred during the corrosive October–March period. Lowest pCO 2 and highest pH occurred during the super-saturated April–September period. The monthly variations in pCO 2 , pH, and aragonite saturation state closely followed the variations in monthly average chlorophyll a . Super-saturated conditions during April–September are likely strongly influenced by photosynthetic uptake of CO 2 during the phytoplankton growing season. The relationship between phytoplankton production, the carbonate system, and aragonite saturation state suggests that long-term trends in eutrophication processes may contribute to trends in ocean acidification in Puget Sound.